1. Field of the Invention
This invention relates to enrichment and separation of isotopes, in particular to laser isotope separation by one-photon excitation of unimolecular reactants to high vibrational states/within the ground electronic state.
2. Description of the Prior Art
Efficient isotope separation is the goal of considerable research worldwide. Pure isotopes are needed to meet a growing demand in scientific research, medicine, agriculture, and power generation. Much success has been achieved in separation based on isotope-selective photo excitation using lasers. Three review articles have summarized this work (V. S. Letokhov and C. B. Moore, Sov. J. Quant. Electron. 6, 129, 259 (1976); N. G. Basov, et al., Sov. Phys. Usp., 20, 209 (1977); and R. N. Zare, Scient. Amer. 236, 2, 86 (1977)).
Direct laser excitation to high vibrational states of molecules was reported by Ambartsumyan, et al., JETP Lett. 15, 237 (1972). Those workers selectively excited the third vibrational state of the HCl molecule using a Q-switched neodymium laser. Based on the dependence of luminescence intensity on the laser excitation wavelength, they determined that the excitation was isotope-selective and speculated about using the process for isotope separation.
Brauman, et al. (Optics Comm. 12, 223 (1974)) proposed unimolecular photoisomerization as an isotope separation method. Their paper treats the subject generally, suggesting neither specific molecules nor laser systems for use in the process. Unimolecular decomposition is an element of laser isotope separation processes disclosed by Kaldor in U.S. Pat. No. 4,000,051, issued Dec. 28, 1976, and by Hartford and Tuccio (Chem. Phys. Lett. 60, 431 (1979)). However, these processes involve multiphoton excitation, which has disadvantages discussed below. The Kaldor process also requires the additional step of attaching thermal electrons to vibrationally excited compounds before decomposition takes place.
Tuccio, et al. (1975 IEEE/OSA Conference on Laser Engineering and Applications, Washington, D.C.) reported on uranium isotope separation by selective photoionization using a two-laser technique. First, uranium which had been thermally excited to the first metastable state was selectively excited to a higher state with radiation from a xenon ion laser. The xenon laser was tuned using a temperature stabilized intra-cavity etalon, which provides only a narrow tuning range. Subsequently, the excited atoms were ionized with radiation from a krypton ion laser. This ionization was accomplished in the intra-cavity mode; i.e., the uranium atoms were excited while in a vacuum chamber intra-cavity to the krypton laser. Intra-cavity excitation was disclosed as part of a laser isotope separation process by Niemann, et al. in U.S. Pat. No. 4,072,590, issued Feb. 7, 1978. Their process, however, requires a "chemical reaction partner" in addition to the gas containing the isotopes to be separated; i.e., their process is bimolecular.
Recently, an intra-cavity dye laser technique was used to isomerize unimolecular reactants (K. V. Reddy and M. J. Berry, Chem. Phys. Lett. 52, 111 (1977)). In that work, methyl isocyanide (CH.sub.3 NC) was excited by direct one-photon excitation to a high vibrational state of the ground electronic state with resultant isomerization to acetonitrile (CH.sub.3 CN).
Unimolecular isomerization has not been considered a promising route to laser isotope separation. In referring to that process, Letokhov and Moore ("Chemical and Biochemical Application of Lasers," Vol. III, ed. C. B. Moore, Academic Press, New York, 1977, p. 72) indicated that most molecules for which unimolecular isomerization has been studied are too large and complex to be likely to exhibit well-resolved isotopic shifts.